Adiabatic lapse is the temperature change that occurs in a fluid as it is adiabatically expanded or compressed. The term is most commonly used to refer to parcels of air in atmospheres, particularly Earth's atmosphere.

Scientific explanation

Adiabatic lapse can be explained by simple fluid mechanics. As a parcel of air travels downward through the atmosphere, it experiences a rise in ambient hydrostatic pressure. The pressure compresses the air, performing mechanical work on it. The energy imparted by the work is stored as thermal energy, causing the parcel to heat up. The converse applies for rising air: it experiences a drop in ambient pressure, does work on the surrounding air, and loses thermal energy.

Air has very poor thermal conductivity, and the bodies of air involved are very large, so transfer of heat by conduction is negligibly small. Thus, the process may be approximated as adiabatic (perfectly insulated).

If the air is saturated with water vapor, a more complex phenomenon arises that retards temperature changes. As it cools, water vapor condenses to form clouds, releasing energy. The actual concentration of water (by mass or any other measure) is small, but because of water's high heat of vaporization, the energy release is very significant.

The relationship between change in altitude and change in temperature is measured as the adiabatic lapse rate, given in units of temperature divided by units of altitude (ex. °C per 1000 feet). In these particular units, the dry adiabatic lapse rate (for relatively dry air) is a constant 2.98. The moist adiabatic lapse rate (for saturated air) is usually near 1.5, but it varies with temperature and pressure.

Significance in meteorology

The adiabatic lapse rate is of critical importance in meteorology. It is often considered together with the gradient of temperature across altitude in stationary air (the environmental lapse rate). If the environmental lapse rate is greater than the adiabatic (i.e. the change in ambient temperature experienced by a moving parcel of air is more rapid than the parcel's own change in temperature), the air is unstable. In this event, the likelihood of thunderstorms or other poor weather is high.

Related topics

• Adiabatic process
• Fluid mechanics

• Adiabatic lapse rate
• Environmental lapse rate